Methods of driving laser diodes, optical wavelength sweeping apparatus, and optical measurement systems

ABSTRACT

An optical wavelength sweeping apparatus having a laser diode with an active region, and a coupled pulse generator is disclosed. The pulse generator is configured and operable to provide current drive pulses of relatively short duration and high amplitude to the laser diode to selectively and rapidly heat the active region and the immediate vicinity and produce a rapid wavelength sweep of emitted optical radiation. Methods of driving a laser diode, and measurement systems are disclosed, as are other aspects.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to and claims priority from U.S.Provisional Patent No. 61/716,410, entitled “Fast Wavelength SweepMethod For Standard Semiconductor Laser Diodes And Optical Fiber SensorInterrogation Systems,” which was filed Oct. 19, 2012, and which ishereby incorporated herein for all purposes.

FIELD

The present invention related generally to sweep methods, system, andapparatus.

BACKGROUND

Change of emitted optical wavelength versus time, e.g. wavelength-sweep,is used within many photonic systems, such as during the interrogationof fiber-optic sensors, various spectrometric applications includingchemical, biochemical and bio-medical sensing, telecommunicationcomponent measurements and testing, and the like. Furthermore, rapidwavelength sweep-rates are often desired. Rapid wavelength sweep allowsfor high-speed interrogation of optical sensors, which is often desiredin dynamic sensing applications. Within multiplexed fiber-optic sensorarrays fast wavelength sweeps provide an opportunity for the timedivision multiplexing of sensors or sensor groups within sensor arrays.Multiplexed sensor arrays often consist of many spectrally-resolvedsensors, such as fiber Bragg gratings, positioned along a single fiber.Interrogation of such arrays often requires high power optical sourcesto compensate for optical losses that are present in such arrays. Inorder to apply time-division multiplexing in such arrays, the wavelengthsweep should be typically completed in less than about 1 μs, moretypically in less than a few hundreds of ns, as the duration of thesweep determines the minimum distance amongst individual sensors withthe same wavelength characteristics.

While there are many known solutions within laser design that canprovide wavelength sweeping capability, the sweeping of wavelengthsassociated with semiconductor laser diodes remain limited to severalrelatively-complex solutions. Traditionally, semiconductortunable/sweepable laser sources incorporate a semiconductor gain medium(e.g., a laser chip) and an external wavelength tunable opticalfeedback. One example is a large external cavity laser diode asdescribed in U.S. Pat. No. 7,212,560. Such tunable/sweepablesemiconductor lasers are usually configured within a Littman-Metcalfcavity arrangement. In this configuration, a laser chip which providesoptical gain is coupled to external mode-selection filtering and tuningelements via bulk optical elements. In most external-cavity approaches,the external mode-selection filter is a diffraction grating that canalso double as a mirror. External optical elements are mechanicallyadjustable to allow for changes in optical feedback geometry, whichfurther leads to a change in the emitted wavelength. Such aconfiguration requires moving micro-mechanical parts that need to beconfigured into a well-defined geometrical configuration. Such tunablesources are therefore complex and expensive for production. Moreover,the sweep-rate may be limited by the mechanical system's Eigenfrequencies. Thus, their wavelength sweep-rate may seldom exceed a kHzrepetition rate.

Several types of fully-integrated tunable laser diodes have also beensuccessfully developed. Such laser diodes do not require any externalmechanical parts, since wavelength tuning is achieved directly bystructures integrated into a laser chip. The first group of tunablelaser diode sources utilizes distributed back reflectors (DBR) locatedat the side or sides of the active region of the laser diode. Thisprovides wavelength selective optical feedback necessary for diodelasing. DBR diodes designs includes a two-section DBR (distributed backreflector) laser diode described in U.S. Pat. No. 6,862,394, athree-section DBR laser diode described in U.S. Pat. No. 6,806,114, anda multi-section DFB laser diode described in US Publication No.2004/0136415. Wavelength tuning of such laser diodes is based on changesin the refractive indexes of one (two-section) or two (three-section)DBR regions of the laser diode. Change in the refractive index of a DBRregion causes a shift in spectral characteristics of a laser diode'soptical feedback, which results in a change of the output wavelength.Changes in the refractive index of the described laser diodes may becaused by electrical current, but can also be achieved by local changesof temperature, with heat-strips located at specific regions, asdescribed in US Publication No. 2010/0309937. While DBR diodes canprovide fast wavelength sweeping, they are relatively complex forproduction, may exhibit mode-hopping, and may be prone to generatingadditional noise. Generally, due to their complex structure, they arenot widely used.

The second group of tunable laser diodes utilizes wavelength tuningstructures that are integrated along (i.e. in parallel with) the diode'sactive region. One example is a tunable twin guide DFB (TTG-DFB) asdescribed in U.S. Pat. No. 7,112,827. Another is a striped heater DFBlaser as described in U.S. Pat. No. 5,173,909. This group of tunablelaser diodes allows for wavelength tunability without mode hoppingeffects. Direct heating can typically induce wavelength sweeps within arange of about 5-10 ms.

Most existing tunable laser diode systems, as described above, havecomplex structures and are consequently not produced in high volumes.The tuning range of such laser sources do not often exceed about 8 nmand the diodes may exhibit mode-hopping effects. Except in the case ofDBR-based diodes, the tuning speed/rate is usually also limited. Thisespecially applies to all systems that utilize temperature-inducedeffects for wavelength tuning.

Due to the relatively high complexity and high costs of known tunablelaser diodes and methods, there is a need for simpler and morecost-effective tunable laser sources. This is especially true in thefield of interrogation of optical sensor(s). Thus, improved methods andsystems to produce wide and rapid wavelength sweeps are sought.

SUMMARY

In one aspect, a method of driving a laser diode is provided. The methodincludes providing a laser diode having an active region, andselectively and rapidly heating the active region and the immediatevicinity by applying current pulses to perform a wavelength sweep ofemitted optical radiation.

In another aspect, an optical wavelength sweeping apparatus is provided.The optical wavelength sweeping apparatus includes a laser diode havingan the active region and an immediate vicinity, and a pulse generatorcoupled to the laser diode configured and operable to provide currentdrive pulses to the laser diode to selectively and rapidly heat theactive region and the immediate vicinity and produce a wavelength sweepof emitted optical radiation.

In another aspect, an optical measurement system is provided. Theoptical measurement system includes a laser diode having an the activeregion and an immediate vicinity, a pulse generator coupled to the laserdiode configured and operable to provide current drive pulses to thelaser diode to selectively and rapidly heat the active region and theimmediate vicinity to produce a wavelength sweep of emitted opticalradiation, an optical diverter coupled to the laser diode, an opticalsensor coupled to the optical diverter, and an observation devicecoupled to the optical diverter.

Numerous other aspects are provided in accordance with these and otheraspects of the invention. Other features and aspects of the presentinvention will become more fully apparent from the following detaileddescription, the appended claims, and the accompanying drawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 illustrates typical dimensions of a laser diode chip.

FIGS. 2A-2F show the simulated evolution of a temperature distributionwithin the cross-section of laser chip over time during a drive currentpulse according to embodiments.

FIG. 3 illustrates a simulated change of the temperature in the centerof the active region and in the immediate vicinity 0.2 μm away from theactive region after pulse excitation of the laser diode.

FIG. 4 represents the test setup for measuring the wavelength sweep of alaser diode.

FIG. 5 shows a reflected optical power vs. time plot of a light pulsereflected from an all-fiber Fabry-Perot etalon with about 4%reflectivity.

FIG. 6 illustrates the wavelength change over time of a laser diodeduring application of short duration pulses.

FIG. 7 illustrates a laser diode spectrum recorded by a gratingspectrometer.

FIG. 8 illustrates a plot of output optical power of a laser diodeduring short duration pulsing.

FIG. 9 illustrates a plot of a spectrum of relative optical power overwavelength of a mode hopping DFB laser diode.

FIG. 10 represents the reflected optical power from an FP interferometerthat is emitted by mode-hopping DFB laser diode.

FIG. 11 shows a plot of reflected optical power over time of a linearwavelength light pulse reflected from an all-fiber Fabry-Perot etalon.

FIG. 12 illustrates the linear wavelength change of a laser diode duringelectric pulse application.

FIG. 13 illustrates an experimentally-determined electrical pulse shapeused to drive a laser diode, which produced substantially linearwavelength sweep.

FIG. 14 illustrates a circuit for discrete pulse-shaping according toembodiments.

FIG. 15 illustrates a wavelength sweeping apparatus including multiplelaser diodes extending the wavelength range according to embodiments.

FIG. 16 illustrates a measurement system adapted to interrogatingoptical sensors (e.g., FBG sensors) according to embodiments.

FIG. 17 illustrates a plot of reflected optical power over time of aFabry-Perot etalon during heating and cooling according to embodiments.

FIG. 18 illustrates a measurement system including a reference adaptedto allow wavelength tuning control according to embodiments.

FIG. 19 illustrates a measurement system including thermal coolingaccording to embodiments.

FIG. 20 illustrates a spectral response plot of a laser diode underpulse-excitation at two different temperatures according to embodiments.

FIG. 21 illustrates a measurement system including an analog-to-digitalconverter (ADC) instead of time-to-digital converter (TDC) according toembodiments.

FIG. 22 illustrates a measurement system including wavelength selectiveelements according to embodiments.

FIG. 23 illustrates a measurement system for a fiber Bragg grating (FBG)array according to embodiments.

FIG. 24 illustrates a measurement system for a FBG array using timemultiplexing according to embodiments.

FIG. 25 illustrates a measurement system for an array of optical sensorsusing a 1×N optical coupler according to embodiments.

FIG. 26 illustrates a measurement system for an array of optical sensorsusing an optical switch according to embodiments.

FIG. 27 shows an experimental setup of a measurement system measuringmultiple optical sensors according to embodiments.

FIG. 28 shows a plot of a change of time difference for a FBG strainchange according to embodiments.

FIG. 29 illustrates a flowchart of method according to embodiments.

FIG. 30 shows averaged and further filtered recorded data according toembodiments.

DETAILED DESCRIPTION

A distributed feedback (DFB) laser diode is a type of single-frequencylaser diode that is currently produced in large quantities (mainly fortelecommunication applications) at low-cost. The ability to use suchlaser diodes in measurement systems that require wavelength sweepingwould thus be highly beneficial from the cost-performance perspective.The tunability of these devices is limited as conventionally operated.Thus, the potential for wavelength sweeping of these devices is alsolimited. Wavelength tuning of a DFB laser diode can be achieved eitherby temperature-cycling or by driven current modulation. Typically, atelecom DFB diode exhibits wavelength shift due to temperature of about0.1 nm/° C. to about 0.2 nm/° C., and due to current modulation ofapproximately 10 pm/mA (depending on the diode design). Whiletemperature-cycling can be used to sweep the diode wavelength overseveral nanometers, such a temperature-controlled wavelength sweepingusually takes several milliseconds or even seconds to conduct, asdescribed in “Rapid temperature tuning of a 1.4-mm diode laser withapplication to high-pressure H2O absorption spectroscopy” (OPTICSLETTERS, Vol. 26, No. 20, Oct. 15, 2001). Drive current modulation canprovide much faster wavelength sweeps, because the applied drive currentcan change rapidly, but conventionally only within a relatively-limitedwavelength sweep range due to a maximum declared continuous DC drivecurrent range. Furthermore, producers of telecommunication DFB diodestend to reduce wavelength dependence on current, since it causes laserdiode wavelength chirp during direct current modulation, which maydegrade telecommunication system performance. A typical DFB diodewavelength can thus be continuously swept by current control for only afew tens of nm, e.g. for 0.1 nm to 0.5 nm as described in US PublicationNo. 2011/0249973. Current wavelength sweeping systems based onconventional DFB diodes can thus provide only limited wavelength sweepranges and/or limited dynamic performance.

In view of the limitations and problems of the prior art, embodiments ofthe present invention provide measurement methods and measurementsystems that can utilize a conventional DFB laser diode (such as awidely-available and low cost telecommunication DFB laser diode) toproduce relatively wide and rapid wavelength sweeps. Embodiments of thepresent methods, systems, and apparatus described herein can also beextended to other types of laser diodes, in particular tosingle-frequency, horizontal-resonator laser diodes. Principles ofembodiments of the invention can be applied to other types ofsingle-frequency, hetero-structural, or quantum-well semiconductorlasers. For example, embodiments of the invention can be applied tolaser diodes with resonators (e.g., horizontal resonators) that haverelatively thin (low thickness) active regions (having thicknesses ofless than 2 μm, or even less than 1 μm, or even less than 500 μm in someembodiments), or to laser diodes having a cross-sectional area of theactive region that is considerably smaller than a cross-sectional areaof the laser diode chip. “Considerably smaller” means 1,000 times ormore smaller, 4,000 times or more smaller, or even 7,000 times or moresmaller in some embodiments, than a cross-sectional dimension of thelaser diode chip. Furthermore, it is preferable for the laser diode chipto have a feedback system that provides for a single-frequencyoperation; otherwise the emitted spectra might include many modes. Inother words, the laser diode is a single-frequency diode.

One class of diodes that are used for the present method are DFB diodeshaving an active region 102 that has a thickness (e.g., verticalthickness) of less than 1 um, an active region cross-section of lessthan 7 um², and a ratio of active region volume to total laser diodevolume of less than 1/300, or even less than 1/1,000 in someembodiments. These low volume ratios indicate low power laser diodeswhich are good candidates for sweeping apparatus, measurement systems,and measurement methods in described herein.

According to another aspect, the measurement system and wavelengthsweeping apparatus intrinsically generates relatively-high optical powerduring a wavelength sweep. Furthermore, embodiments of the measurementsystem and wavelength sweeping apparatus can be configured tointerrogate and time-multiplex several types of optical sensors, such asfiber Bragg gratings, Fabry-Perot interferometers, or otherspectrally-resolved sensors or optical devices, rapidly and at low cost.

It is generally well-known that changes in the drive-current of a DFBlaser will affect a wavelength of the laser diode emission output. Thischange in output wavelength (“wavelength shift”) is generally referredto as wavelength chirp. It can be predominately attributed to arefractive index change of the resonator of the diode caused by aheating of the active region of the diode that contains the Braggstructure. The wavelength shift induced by the drive current supplied tothe laser diode is, however, limited by the maximum declared continuousDC drive current of the diode. The maximum declared continuous DC drivecurrent of the diode is defined by the thermal limitations of thesemiconductor material of the diode. A typical telecom DFB laser diodepermits a maximum declared continuous DC drive current within a range ofbetween 50 mA and 100 mA. A typical DFB laser has optical emissionpower, typically between 1 mW and 10 mW, and also exhibits drivecurrent-induced wavelength shift of the order of 10 pm/mA. Thisconventionally limits the wavelength sweep range to between 0.5 nm and 1nm, even in extreme cases. Of course, this range is not sufficient forwavelength sweeping sensor interrogation methods.

The “active region” denotes a space within the laser diode where opticalamplification occurs. Active region is where a majority of the carrierrecombination takes place (i.e. where holes and electrons recombine). Inlaser diodes this region may be defined, for example, by a physicalfeature(s), such as a double heterostructure, i.e., a sandwich of threematerials, wherein the middle material is very thin and forms the activeregion. The vertical dimension of this active region is defined bythickness of the material in the middle of the heterostructure sandwich.Multiple quantum well lasers have a similar structure, except the activeregion (material in the middle of “sandwich”) may not be uniform butrather may be composed of many (typical 3-10 nm thick layers) ofmaterials with alternating composition. The total thickness of thisactive region composed of alternating, ultra-thin layers is againtypically less than about 1 μm thick, more typically about 500 nm thickor less.

Thus, typically, the active region has a submicron thickness or even athickness within the range of a few hundred nanometers or less, in orderto confine the recombination the charge carriers to a smaller volume andconsequently provide the high-density charge carriers required foroptical gain generation. The active region may be designed in the formof semiconductor heterostructure, multiple heterostructures, multiplequantum well, or similar semiconductor designs that confine the chargecarrier recombination into a relatively small volumes. When driving theelectric current through the laser diode, a great majority of chargecarrier recombination takes place within the active region of the laserdiode, which also restricts the majority of the voltage drop to the sameactive region. Only a small fraction of the total voltage drop developsover the rest of the laser diode chip.

Furthermore, as shown in FIG. 1, the active region 102 is embeddedwithin the bulk of the semiconductor material of the laser diode chip100, which has relatively low electrical and thermal conductivity. Whenthe electric current is driven through the semiconductor laser diodechip, most of the voltage drop is developed over the active region. Thiscauses dissipation of heat from the same active region. The heatgeneration during the application of the drive current is, thus,predominantly limited to the active region of the diode. Accordingly,the generated heat is conducted through the rest of the laser chip tothe surrounding diode case and is further dissipated to the surroundingenvironment. The steady-state temperature of the active region and thelaser chip is a main factor that determines maximum declared continuousDC drive current, and thus maximum wavelength shift induced and achievedby direct current (DC) excitation. Thus, the DC driven, steady-statetemperature of the laser chip depends on the thermal conductivity of thelaser chip, diode case, junction between the laser chip and diode case,heat conduction from the diode case, and other factors that affectthermal resistance between the active region and surroundingenvironment.

A change in the emission wavelength is a consequence of atemperature-induced refractive index change of a semiconductor materialthat constitutes an optical feedback structure of the laser diode. In aDFB diode, the optical feedback structure may be a periodic formation,such as a Bragg grating or other optical formation. The optical feedbackstructure, which is usually located in the immediate vicinity (e.g.,within about 500 nm or less) of the active region 102, and may be withinabout 120 nm to about 200 nm of the active region 102 in someembodiments, affects the optical field in the active region 102 throughevanescent field interaction. In some embodiments, the optical feedbackstructure may be part of the active region 102. Heating of the activeregion 102 or the entire diode thus results in associated feedbackstructure heating, which causes a wavelength shift of emitted radiationdue to its refractive index change.

According to one or more embodiments of the present invention, an activeregion of a laser diode and its immediate vicinity are selectively andrapidly heated. This selective and rapid heating is provided byapplication of short-duration, high-amplitude pulses. This allows forthe performance of a fast wavelength sweep of emitted optical radiation.Complete wavelength sweeps in less than 500 ns may be achieved. In otherembodiments, even faster sweeps, i.e., in less than 250 ns, or even inless than 100 ns, are possible by shortening of the pulse duration whilesimultaneously increasing the driven current amplitude. “Selectivelyheating” or “selective heating” as used herein means preferentiallyheating of the active region 102. Selective heating ensures thatimmediately after driven current pulse termination, that 90%, 95%, oreven 99% of the total diode chip bulk volume remains at temperature thatis less than 7 degrees C., less than 3 degrees C., or even less than 2degrees C. above the chip temperature before the current pulseinitiation.

Selective heating of the active region 102 of the laser diode 100 andthe immediate vicinity thereof may be achieved by the application of arelatively short-duration, and high-amplitude drive current pulses. Asrecognized by the inventors herein, the selective heating of the activeregion and the immediate vicinity is possible due to the confinement ofthe voltage drop largely to the confines of the active region andimmediate vicinity, which has relatively small dimensions, especially incomparison to the overall dimension of the laser chip. Accordingly,application of a high amplitude drive-current pulses of relatively shortduration can thus selectively deliver heat only to the active region ofthe diode (and optical structures in a directly surrounding vicinity),thereby causing rapid and selective heating (increase in temperature).The selective and rapid heating comprises application of short-durationcurrent drive pulses having a pulse duration, and amplitude sufficientlylow to prevent a rise in an average bulk temperature of the laser chipabove 30 degrees C., or even less than 10 degrees C., or even less than7 degrees C., or even less that 3 degrees C. in some embodiments. Fromanother viewpoint, selectively means that the temperature of activeregion 102 is at least 30 degrees C. higher, or even 40 degrees C.higher, or even 70 degrees C. higher at the end of a single currentpulse than before application of the current pulse, while the averagetemperature of the diode chip 100 is increased by less than 10 degreesC., or even less than 7 degrees C., or even less than 5 degrees C., oreven and less than 3 degrees C. in some embodiments.

In accordance with an aspect of the invention, a rapid heat-up sequenceof the active region is provided, but where the bulk of the chip remainsrelatively close, as described above, to the average environmentaltemperature. The applied current pulse may be spontaneously followed bya prompt cool-down process as the heated active region is rapidly cooledby the surrounding cooler bulk mass of the chip. The active regionresides within the bulk non-heated (relatively colder) laser chip andthe heat conduction process provides dissipation of the heat generatedacross the entire volume of the laser chip.

Low duty ratio current pulses should be used to prevent any rise in theaverage temperature of the laser chip 100. A “duty ratio” as used hereinis a ratio between a duration of the pulse (Lpul) and a sum of theduration of the pulse (Lpul) and pause (Lpau) between the pulses.Duty ratio=Lpul/(Lpul+Lpau)  Eqn. 1

Short-duration, high-amplitude, laser diode drive-current pulses havinga relatively low-duty ratio can thus provide rapid and selective heatingas well as cooling cycles within the active region 102 and its immediatevicinity, while an average temperature of the laser chip 100 can remainrelatively close to an average environmental temperature or asteady-state operating temperature. Duty ratios of less than about 1/10,or even less than about 1/30, or even less than about 1/100, or evenless than about 1/1,000 may be used.

The maximum allowable amplitude of the driven current pulse when appliedin a low duty ratio pulse excitation is considerably higher than in thecase where continuous DC drive current excitation used in the prior art.Pulse amplitudes can usually be about 5 times to about 100 times largerthan a maximum declared continuous DC drive current. The maximumdeclared continuous DC drive current is a limit on the driven currentdeclared or specified by the manufacturer for continuous DC use withoutthermally damaging the laser diode 100. In other embodiments, pulseamplitudes may be 5 times larger, or even 5 times to 30 times larger, oreven 5 times to 50 times larger, or even 100 times larger or more, oreven 5 times to 300 times larger or more than the maximum declaredcontinuous DC drive current in some embodiments.

The selective and rapid heating may be accomplished by causing aduration (e.g., a pulse width) of the short-duration driven currentpulse to be less than about 50 μs, or even less than about 10 μs, oreven less than about 5 μs, or even less than about 2 μs, or even lessthan about 1 μs, or even less than about 500 ns, or even less than about100 ns in some embodiments. The shorter the pulse duration, the morerapidly the sweep may be accomplished. Limiting the pulse duration mayprevent any significant rise in a bulk temperature of the laser chip.The combination of relatively short pulse duration and relatively higherpulse amplitude of the driven current pulses may also be used. Theduration and amplitude may be selected in a way that a peak temperatureof the active region remains below semiconductor degradationtemperature, i.e., a temperature at which optical gain and consequentlythe output power of the laser diode 100 becomes degraded below a usefulthreshold. Since the peak temperature of the active region 102 dependson the energy delivered by the current pulse to the active region 102and the initial temperature of the laser diode chip 100, the appliedpulse duration and pulse amplitude may be selected taking into accountthe initial temperature of the laser diode chip. Typical peak increasesof a temperature of the laser diode chip can be in the range between 30degrees C. to 100 degrees C., or even more than 130 degrees C. when thelaser diode chip is initially at room temperature. If the diode chip isactively cooled, such as with a active cooling device (e.g., athermoelectric cooler) this range can be further extended. Typically, arange of wavelength operation may be extended and correlates to thereduction of the diode chip's environmental temperature. Furthermore,average bulk laser chip temperature increase due to the single-pulseapplication may be limited with such cooling to below 3 degrees C., oreven below 0.5 degrees C. in some embodiments. During current pulseapplication, the increase of the average bulk chip temperature can begreater, however average temperature increases of greater than about 30degrees C. are to be avoided by providing pulsed current to the diode100 having short duration and low duty ratios, cooling, or combinationsthereof.

Rapid temperature cycling of the active region 102 and its vicinity,achieved by short duration, high-amplitude pulse excitation, may lead torapid changes of optical parameters within the optical feedback systemof the laser diode chip, as already explained above. These rapid changeslead to rapid sweeps of the wavelength output of the laser diode chip.For example, in a DFB diode the active region 102 also confines theoptical field and provides guidance between the front and back sides ofthe laser diode 100. Furthermore, in the immediate vicinity of theactive region 102, an optical feedback structure comprising a periodic(Bragg) structure is located that provides optical feedback for laseroscillations that further determine diode emission wavelength. Rapidtemperature variations of the active region and its immediate vicinitythus cause rapid changes in the refractive index of the material thatforms the optical feedback structure comprising the Bragg structure,which leads to rapid changes in the emission wavelength of the DFB laserdiode.

Numerical and experimental investigations were performed on varioustypes of commercial telecom DFB laser diodes to determine the drivencurrent amplitude limits when operated in low-duty cycle ratio regime ofoperation and associated wavelength sweep-ranges achievable. Detailedexamples of these investigations are given below.

Finite-element modeling of the thermodynamic behavior was performed fora typical laser diode, in order to assess temperature changes that wouldbe feasible in a typical active region of a DFB diode, while alsopreventing permanent damage to the diode. The DFB laser diode wasmodeled as an InP (Indium phosphide) laser diode chip. It was assumedthat the laser diode chip had a thermal conductivity of 40 Wm⁻¹K⁻¹ andspecific heat of 3100 Jkg⁻¹K⁻¹ with dimensions of 250 μm×300 μm×85 μm,as shown in FIG. 1. The modeled active region 102 was made from InGaAsP(Indium Gallium Arsenide Phosphide). It was assumed for the modeledactive region 102 that the thermal conductivity was 6 Wm⁻¹K⁻¹ and thespecific heat was 3100 Jkg⁻¹K⁻¹, and the modeled dimensions were 5μm×300 μm×0.5 μm, as shown in FIG. 1.

It was further assumed that the diode chip is fixed into the case havinga constant temperature at 300 degrees K with air as the surroundingmedium with a free convection coefficient of 20 W/m²K. FIGS. 2A-2F showvarious plots of simulated temperature distribution evolution over timein a transverse plane (along section 2-2 of FIG. 1) of the laser diodeover time-steps corresponding 20 nm, 100 nm, 200 nm, 300 nm, 500 nm, and1,000 nm after turning the current pulse on and starting to deliver 6 Wof heating power to the active region 102 and then rapid coolingthereof. The model assumed a typical heating power of 6 W that isreleased within the active region 102. 6 W of heating power can be, forexample, generated by a voltage drop of 2 V over active region and drivecurrent amplitude of 3 A. The diode cross-section had dimensions aspresented in FIG. 1. The duration of the simulated pulse was 250 ns inthis model. The various plots illustrate that the temperature risesrapidly upon imparting the current pulse, but the temperature is onlylocally increased. Similarly, the plots illustrate that the temperaturerapidly returns to near that of the bulk of the diode chip 100.

FIG. 3 shows a plot of temperature versus time illustrating dependencein the center of the active region 102 and the temperature of a regionin the immediate vicinity (0.2 μm away) from the top boundary of theactive region 102. After termination of the pulse (at about 0.24microseconds), a formed heatwave continues to propagate through the chip100 towards the outer surface of the chip 100, while a temperature ofthe active region 102 starts to fall, almost exponentially, with atime-constant of about 100 ns. A full temperature recovery of the activeregion 102 to be substantially the same as the surrounding temperatureof the laser chip 100 takes approximately 10 to 20 μs. Substantially thesame, as used in this context, means within about 99% or less of atemperature of the bulk temperature. FIG. 3 also illustrates a minimumnecessary pulse-off state to prevent a significant rise of a temperatureof the laser chip 100 above the diode case temperature during aninstance of continuous pulse excitation.

As shown by this numerical model simulation, large and rapid changes ina temperature of the active region 102 and its immediate vicinity arepossible by application of high-peak amplitude, short-duration, andlow-duty ratio pulse excitation of a typical DFB diode (e.g., a telecomDFB diode). An absolute upper temperature of the active region 102 maybe defined by a loss of semiconducting properties of the active region102 (and associated optical gain decrease) and temperature-induceddegradation of the semiconductor material, which may lies above about140 degrees C. Since a typical change in temperature of 1 degree C. in amodeled InGaAsP diodes causes a 0.05 nm to 0.10 nm wavelength shift ofthe laser emission, wavelength shifts of greater than 5 nm, or evengreater than 7 nm, or even greater than 10 nm or may be achieved. Thefull temperature change range, and thus wavelength sweep-range, could befurther extended by lowering the initial (case) temperature of the laserdiode such as, for example, by utilizing a thermoelectric cooler.

Several experimental example telecom laser diodes were measured in ameasurement system 400 to confirm the above assumptions and simulations.A first example of a measurement system 400 is shown in FIG. 4. Themeasurement system 400 includes a laser diode 401 (e.g., a DFB laserdiode) which is undergoing the test, an optical coupler 404 (e.g., a 3dB optical coupler), an optical sensor 405, which in this case iswavelength reference such as a reference all-fiber, Fabry-Perot etalon(e.g., having a 5% average reflectivity), a current pulse generator 406(such as a programmable current pulse generator), such as for example apulse generator constructed from microcontroller, MOSFET driver, andMOSFET transistor which all together act as fast current switch to drivethe laser diode 401, and an observation device 408 that allowsprocessing and/or observation of constantly varying signals (e.g.,currents or voltages), and which may plot or display the processed dataas a two-dimensional graph of one or more electrical currents orpotential differences. The data may be displayed using the y axis, andplotted as a function of time (x axis). One example of an observationdevice 408 is an optical oscilloscope 408 (e.g. a HP 83480A modeloscilloscope with optical module HP 83485B), and various sections ofsingle-mode optical fiber 410, 412, and 414 at each branch of theoptical coupler 404. A standard single mode optical fiber, such asSMF-28 ® optical fiber available from Corning Incorporated may be used.Other optical fibers may be used.

The optical sensor 405, in this example is a reference Fabry-Perotetalon, and was created by the formation of a semi-reflective mirrorwithin optical-fiber that was placed about 0.85 mm away from theflat-cleaved fiber-end. In-fiber mirror formation is described in detailin “Low-loss semi-reflective in-fiber mirrors” (Cibula E, Donlagic D.,Optics Express, Vol. 18, Issue 11, pp. 12017-12026, 2010). The all-fiberFabry-Perot etalon had free spectral range (FSR) that corresponded to0.96 nm at 1550 nm and mirror reflectances of about 3.2% for the firstmirror (air cavity within optical-fiber) and 3.6% for the second mirror(flat cleaved optical-fiber).

The pulse generator 406 was set to trigger pulses having a duration of250 ns, an amplitude of about 2 A with a repetition rate of about 10kHz. After the optical pulse was triggered, it traveled throughsingle-mode optical-fiber section 410 to the optical coupler 404, thenfurther along the single-mode optical-fiber section 412 to the opticalsensor 405 (e.g., a Fabry-Perot etalon). The optical pulse was thenreflected from the optical sensor 405 and was further guided by thefiber section 412 to the optical coupler 404 where it was split into twobranches using the 3-dB division ratio of the optical couple 404. Thebranch containing the single-mode optical fiber section 414 led theback-reflected pulse to the observation device 408, which furtherdisplays the reflected pulse power versus time characteristics. Thepulse duration was gradually increased while observing theback-reflected signal from the optical sensor 405. The pulse durationwas limited to a particular value when any further prolongation of thepulse duration caused the temperature of the active region (e.g., 102)of the laser diode 401 to rise to the level that lowered the gain andconsequently affected the diode's output power below a useful threshold.

A wavelength change versus time during pulse duration was reconstructedto form a response of the sensor 405 (e.g., the Fabry-Perot etalon).Each full-fringe, e.g., each new peak within the back-reflected spectrarepresents a wavelength change that corresponds to the free spectralrange of the reference Fabry-Perot etalon (e.g., 0.96 nm). Thereconstructed emitted wavelength change during the pulse durationobtained from the data in FIG. 5 is represented in FIG. 6. It is clearfrom FIG. 6 that a wavelength change (wavelength sweep) of greater than10 nm is achieved in less than 250 ns.

The experimental results shown in FIG. 5 to FIG. 7 were obtained byexperimentally testing a typical commercial DFB laser diode typeOSMLDP-D513BF2G, produced by SICHUAN OSEMOS ELECTRONIC TECHNOLOGY INC,China. FIG. 5 represents the back-reflected optical power over the totalpulse duration, which corresponded to 250 ns.

In order to further confirm the above result, the oscilloscope acting asthe observation device 408 was replaced by a grating spectrometer. Thespectrometer, acting as the observation device 408, confirmed that theaverage generated optical power spectrum exceeded a 10 nm width, asshown in FIG. 7. Furthermore, the output power versus time of the diode401 was measured, which indicated the peak power of the laser diode 401was within the range of 140 mW (at the beginning of the pulse) and at aminimum power (at the end of the pulse) corresponding to about 10 mW (atend of the pulse when the temperature of the active region 102 was thehighest), as shown in FIG. 8. Average optical power is around 60 mW whenoperated in pulsed mode. The laser diode 401 had a declared maximumcontinuous nominal output power of 2 mW or less when powered withcontinuous DC power. Such high optical power generated by the presentlaser diode operating method is advantageous for all measurement systemsthat suffer from high optical losses. In particular, the generation ofhigh optical power might be of special importance in multiplexed sensormeasurement systems, such as time division multiplexed measurementsystems, where many optical sensors reside along a single optical fiberline, and thus each optical sensor contributes to loss within themeasurement system. By applying selected amplitude and pulse duration ofa current pulse to the laser diode, method embodiments of the presentinvention may produce peak optical power at the output of the laserdiode that is 5 times higher, or even 10 times higher, or even timeshigher, or even 50 times higher, or more than a maximum declaredcontinuous DC drive current declared by the manufacturer.

The change in output optical power of the laser diode duringshort-duration, high-current pulse is a good indication of practicalcurrent amplitude and duration limits that can be imposed. Overheatingof active region is indicated by a rapid drop in the emitted opticalpower. Therefore, observation the optical power at the end of theapplied current pulse can be used as an indicator of maximum electricalpulse amplitude at give pulse duration. Too high of an amplitude willoverheat the active region.

The laser diode under test (e.g., a Osemos OSMLDP-D513BF2G) exhibited acontinuous wavelength shift over the pulse duration. However, it shouldbe noted that this is not the case with all commercial DFB diode types.High current is also associated with high optical gain and temperaturechange within an active region, which can lead to mode hopping. Such anexample of mode hopping is shown in FIG. 9. In this example, the laserdiode under test was LD-PF2-D5102-1GR by WAVE SPECTRUM, China. The laserdiode first shifts its wavelength for 3.2 nm, hops for 1.2 nm, and thenfinally further shifts continuously for another 3 nm. Furthermore, thelaser diode exhibited smaller emitted maximum optical power compared tothe previous example laser diode. In FIG. 10, for example, emitted lightfrom mode hopping in the diode was reflected by the optical sensor 405(e.g., an all-fiber Fabry-Perot interferometer). Mode hop is clearlyvisible as discontinuity in recorded time response at about 120 ns inFIG. 10. During the investigation, the inventors discovered that themode hopping occurred consistently at the same wavelengths in the caseof a particular diode, therefore even laser diodes that exhibitmode-hopping can be used in certain applications, particularly whenappropriate signal-processing is used.

In general, it is desired to use a laser diode 401 that exhibits acontinuous wavelength shift over the entire pulse current and durationspan. Furthermore, it should be stressed that some telecom laser diodesexhibit relatively lower wavelength shift under the described pulseexcitation. This may be due to the intrinsic design of the laser diode,which aims to suppress wavelength shift caused by any current changes.Wavelength modulation or chirp caused by modulation of the drive-currentis, in general, an undesirable characteristic within telecom systemssince it increases adverse effects of chromatic dispersion. Certaintelecom diode designs therefore tend to introduce various designmeasures to limit any wavelength sensitivities of the laser diode tocurrent modulation. Such sensitivities can, in such cases, limitachievable wavelength sweep-range. It was noticed, for example, thatlaser diodes intended for direct modulation and certain diodes for usein DWDM systems, such as produced by Oclaro Inc, USA, exhibited muchlower wavelength shift than most of the other tested diodes.

In another aspect, wavelength versus time λ(t) characteristics ofemitted optical radiation follows the temperature versus timecharacteristics of the active region 102. When short duration,rectangular-shaped current pulses are used for laser diode excitation,the temperature and consequently the wavelength versus time λ(t)characteristics are nonlinear, e.g. approximatelyexponentially-asymptotic, as shown for example in FIG. 6. Mostapplications, especially measurement applications, however it is desiredto have a substantially linear wavelength change versus time. In suchcases, proper shaping of the current pulse can be applied to achieve alinear wavelength change versus time. In accordance with another aspect,linearization of the wavelength versus time λ(t) characteristics may beaccomplished by current pulse shaping that can be achieved by theapplication of a pulse generator 406 coupled to the laser diode 401 thatmay be fast programmable. Pulse shaping comprises shaping the pulse tosubstantially linearize the change in output over a wavelength range.

In one embodiment, system modeling may be applied to determine therequired current versus time characteristics, i.e., the pulse shape,which will provide a relatively linear wavelength vs. time sweep, aheuristic approach can also yield good results. In another embodiment,heuristic linearization of the wavelength versus time characteristics isshown in FIG. 11 and FIG. 12. The same laser diode 401, as was used inprevious tests (e.g., Osemos OSMLDP-D513BF2G), was excited by the shortduration, current pulse shown in FIG. 13. The driven pulse supplied tothe laser diode 401 had a duration of about 300 ns or less, and anaverage current amplitude of about 1.5 A. The driven current pulseshape, in the depicted embodiments, has a shape that increases inamplitude over the length of the pulse. The amplitude may increase byabout 25% or more over the pulse length. For example, the depicted pulseis driven to increase rapidly to about 1 A at the beginning of the pulse(ignoring transients) and then increased to about 2 A over a pulselength of about 300 ns. The response of the sensor 405 (e.g., aFabry-Perot etalon) is shown in FIG. 11. A reconstructed wavelengthshift versus time plot is shown in FIG. 12, where a nearly linearrelation between wavelength (in nm) and time (in ns) was obtained. Anapproximately linear wavelength shift of greater than 10 nm was measuredin less than 300 ns. Approximately linear as used in this context meansvarying less than about 5% from a linear curve fit of the data.

In some embodiments, it is desired to drive the current pulse applied tothe laser diode with an amplitude and pulse duration selected in a wayto provide a ratio of a peak optical power at a beginning of the pulse(discounting transients) to a minimum optical power at the end of thepulse duration of less than 15:1, or even less than 10:1. The ratio ofthe emitted peak pulse power to the optical power at the end of a pulseindicates appropriateness of pulse duration and current pulse amplitudeselection. To prevent unnecessary active region overheating and tomaintain acceptable emitted pulse optical power range this rate shall belimited to not more than ratios above. Extension of pulse or amplitudebeyond these limits may unnecessarily thermally stresses the laser diodeand can lead to life-time reduction or diode distraction.

A pulse generator 406 (e.g., a fast programmable pulse-generator)operable to produce a drive current having a desired pulse shape todrive the diode 401 can be implemented in many ways. One embodiment, thepulse generator 406 includes generation (e.g., an increase) of currentamplitude over a number of discrete time intervals, such as by the pulsegenerator 406 shown in FIG. 14. In particular, a series of N transistors1441-1443 may be set or provided in a circuit in parallel, as shown, andelectrically connected to the laser diode 1401 through N resistors withdifferent resistances 1451-1453. The pulse is then generated bysequential switching of the N transistors 1441-1443 in on-state andafter switching the last transistor 1443 in the on-state, alltransistors 1441-1443 are switched off to terminate the current pulse.

Each resistor 1451-1453 includes a resistor magnitude that determinesthe current that is added to the total current flowing through the laserdiode 1401. By selecting appropriate resistor values, the desiredcurrent versus time characteristics, i.e., pulse shape, can be obtained.While such an approach provides discrete increases in drive current, thetotal current is partially smoothed out due to the parasitic and/orpossible intentional inductances that are or could be present in theelectrical circuit. Since the time between the switching of theneighboring transistors should be considerably shorter than thetime-constant of the laser diode 1401, the thermal inertia of the activeregion 102 will also prevent discrete hops in its temperature,consequently leading to a relatively continuous and smoothwavelength-change versus time. For example, a chain of eight transistorswas used in one example to obtain the drive current shown in FIG. 13while the resistance values 1451-1453 were set heuristically to obtainlinear wavelength versus time response. Even through the generation ofthe drive current to the laser diode 1401 over eight discrete stepscaused discrete changes in the current amplitude, the final wavelengthshift was continuous and substantially linear, likely due to the thermalinertia of the active region 102, as explained above.

In addition, a reference all-fiber Fabry-Perot etalon or anotherwavelength reference could be permanently included within a measurementsystem to provide feedback and adjustment to dynamic pulse-shapingeither to compensate for changing diode characteristics or to providemore versatility when shaping wavelength versus time characteristics.Several examples are described herein.

Other types of custom or integrated pulse generators 406 for pulseddriving of laser diodes 401 can be used instead of the discretetransistors switches described above. Many cost-efficient integratedsolutions, such as drivers for laser projectors, Blue-Ray discplayers/recorders or even digital-video-disc (DVD) players can besuccessfully utilized as a pulse generator 406 to drive a laser diode401 within a measurement system 400 described herein. Such a pulsegenerator 406 often allows for current programming, which can beutilized to adjust the output wavelength versus time characteristic ofthe laser diode 401 during its operation, as described above. Pulsegenerators that use passive or active analog circuits such asresistance-inductance-capacitance (R-L-C) circuits, can be also used forsupplying appropriately shaped current pulses. These can be alsocombined with combinations of discrete switches to achieve control ofpulse shape. As shown in FIG. 15, in order to provide and cover an evenbroader wavelength sweep range, the optical sweeping apparatus 1500 mayinclude multiple laser diodes 1501A-1501C with different initialwavelengths and/or wavelength spans. The laser diodes 1501A-1501C can beconfigured in parallel by using a combining device 1504 coupled to thelaser diodes 1501A-1501C. The combining device 1504 can be, for example,a coupler, a circulator, or DWDM combiner. This combining device 1504may have as many input ports or channels 1510A-1510C as the number oflaser diodes 1501A-1501C within the optical sweeping apparatus 1500. Theoptical sweeping apparatus 1500 may include one output port 1510D (showntruncated) that may further connect to an optical part (e.g., anotheroptical diverter coupler) of the signal processing unit (not shown, butmay include, for example, 404, 412, 414 and 408 as in FIG. 4). Eachlaser diode 1501A-1501C may be pulsed in consecutive order to cover abroad wavelength range. The pulses may be generated by a pulse generator1506 coupled to and adapted to supply drive current pulses to each ofthe laser diodes 1501A-1501C. For example, the first laser diode withthe shortest initial wavelength 1501A may be swept from its initial toits maximum wavelength. This maximum wavelength should coincide with theinitial wavelength of the next laser diode 1501B. When the sweep of thefirst laser diode 1501A is finished, the second laser diode 1501B isswept across the second wavelength range abutting the first wavelengthrange in the same manner as the first laser diode 1501A, and so on. Theresult should be a broad and continuous wavelength sweep across abroader wavelength range that well-exceeds a wavelength sweep rangeachievable by a single laser diode.

In another embodiment, a wavelength sweep range capability of theoptical sweeping apparatus 400 having the laser diode 401 can beextended by, in one embodiment, actively lowering a bulk temperature ofthe laser diode 401. Lowering the bulk temperature of the diode 401reduces an initial temperature of the active region 102 of the diode 401and consequently the initial emission wavelength. This increases totalavailable temperature sweep span of the active region 102 and theimmediate vicinity thereof. The upper temperature limit of the activeregion 102 is limited by semiconductor properties, as explainedpreviously. Lowering of a temperature of the laser diode 401 can be, forexample, achieved by application of a thermoelectric cooler 415 inthermal contact with the laser diode 401 as shown in FIG. 4. Thethermoelectric cooler 415 may be either integrated within the same caseas laser diode 401 or can be provided as a separate element thermallycoupled to the diode case that cools down portions of or entire diodecase.

In summary, by the driving continuous wave-laser diodes, such asconventional DFB laser diodes (e.g., conventional telecom DFB laserdiodes), with relatively high-peak-amplitude, relatively short-durationpulses that have a time duration of the order of a time constant of theactive region 102, one can effectively sweep a laser diode emissionoutput wavelength over a broad wavelength range. The wavelength rangesweep can exceed 10 nm. Such wavelength sweeps can be accomplished overa short period of time that is typically less than 1 μs, or even lessthan 0.5 μs, or even less than 400 ns, or even less than 250 ns in someembodiments. To allow for continuous operation of laser diodes 401within such a pulsed regime, low-duty ratio current pulses may be used.This operates to prevent any appreciable rise in average or bulktemperature of the laser diode 401. A typical duty ratio of less than0.05, or even of less than 0.01, or even less than 0.001 in someembodiments, may be used. Duty ratio is as described in Eqn. 1 above.Furthermore, typical pulse repetition rates of between about 10 KHz andabout 100 kHz can be achieved with pulse duration below 1 μs, whileusing current pulses with amplitudes that typically exceeded the maximumdeclared continuous DC drive current by 5 times or more, 10 times ormore, or even 50 times or more in some embodiments.

In accordance with one or more embodiments of the present invention, astandard telecommunication diode, such as a telecommunication DFB diode,intended for single wavelength operation can be used to provide veryfast and high-optical power wavelength sweeps over a wavelength range ofgreater than 5 nm, greater than 8 nm, or even greater than 10 nm in someembodiments. A standard telecommunication diode can thus be used,according to embodiments of the present invention, in variousapplications currently requiring much more complex tunable lasersources. Furthermore, the wavelength versus time characteristics of thesweeps can be linearized or otherwise shaped by proper programming ofthe current pulse amplitude versus time characteristics, i.e.,controlling the pulse shape. These very fast and short durationwavelength sweeps can be used for time division multiplexing and otherapplications requiring both short duration wavelength-swept opticalpulses, as well as relatively high-optical power. Optical power ofgreater than 100 mW is possible, with average optical power being around60 mW when operated in pulsed mode.

Furthermore, embodiments of the wavelength sweeping apparatus 400 may beused to provide wavelength sweeping in optical sensor interrogationsystems that are adapted for the interrogation of spectrally-resolvedoptical fiber sensors. In particular, optical sensor interrogationsystems in accordance with embodiments of the invention may be used onFiber Bragg Gratings (FBGs), however the principles and systemsdescribed further below can be with or without modifications applied toother similar spectrally-resolved optical sensors, such as properlydesigned fiber Fabry-Perot sensors, Michelson and other interferometers,chemical/biochemical sensors, and the like.

One optical interrogation system 1600 is shown in FIG. 16 and includes alaser diode 1601 (e.g., a DFB laser diode), a pulse generator 1606(e.g., a programmable current pulse generator), an optical diverterdevice 1604, such as a fiber-optic coupler, a circulator, a beamsplitter, or similar diversion device, and an optical observation device1608. The optical observation device 1608 may include an opticalreceiver 1616 that may further include a photo-detector, atrans-impedance device or amplifier 1617, decision logic device 1618such as a comparator or limiting-amplifier, a time measures subsystem1619, such as a time-to-digital converter (TDC), and a processing unit1620. The optical interrogation system 1600 includes various lengths ofoptical fiber 1610, 1612, 1614 that optically interconnect the opticalinterrogation system 1600 with at least one optical sensor 1615, such asa spectrally-resolved optical sensor. One example of aspectrally-resolved optical sensor is a fiber Bragg grating. Other typesof optical sensors may be interrogated by the optical interrogationsystem 1600.

The operation of the optical interrogation system 1600 will now bedescribed in detail. The processing unit 1620 triggers the pulsegenerator 1606 by signal in electrical line 1622, in order to generatean electrical current drive pulse that drives the laser diode 1601 bythe electrical connection 1623, which further generates a short durationoptical-pulse that varies its wavelength over time over a wavelengthoutput range. At the time of initially generating the optical pulse, thetime measures subsystem 1619 is also triggered through the data line1624 by the processing unit 1620. The optical-pulse propagates throughthe length of the optical fiber 1610, the optical diverter 1604 and thelength of optical fiber 1612, until it reaches sensor 1615 (e.g., FBG).The FBG is transparent to all wavelengths except to a characteristicwavelength, which indicates the state of the FBG. As widely-known in theart, a FBG's characteristic wavelength depends on theinitially-inscribed period of the FBG, and the concurrent strain andtemperature imposed on FBG. In operation of the FBG, a change in strainor temperature causes a shift in the FBG's characteristic wavelength.The FBG thus only reflects-back a narrow part of the incoming pulse thathas a wavelength equal to the characteristic wavelength of the FBG.Since pulse-wavelength changes proportionally over time, the time atwhich the back-reflection from FGB occurs depends on the FBG'scharacteristic wavelength. The reflected pulse propagates back along thelength of the optical-fiber 1612 into the diverter device 1604, whichthen splits part of the incoming light into the section of optical fiber1614, which is further connected to the optical-receiver 1616. Theoptical receiver 1616 converts the incoming optical signal into anelectrical signal, which travels over an electrical line 1625, to thetrans-impedance amplifier 1617 where it is amplified. Thetrans-impedance amplifier 1617 is further electrically connected byconductor 1626 to the decision logic device 1618 (e.g., a comparator)which may function to compare the amplified electrical pulse at areference level. The signal at the output of the decision logic device1618 is a square pulse that further triggers the time measures subsystem1619 by an electrical connection 1627. The time measure subsystem 1619determines time T₀ between the start of the initial pulse generation andthe arrival of the back-reflected pulse to the optical receiver 1616,and sends the result over data the line 1628 to the processing unit1620. The processing unit 1620 further subtracts from this time T₀, atime that is required for the optical pulse to transverse the path fromthe laser diode 1601 to the sensor 1615 (e.g., FBG) and back to theoptical receiver 1616 and the propagation delay T_(PD) generated by theelectronic part of the receiving electronics (trans-impedance amplifier1617, decision logic device 1618, and electrical connections):T=T ₀ −T _(PD)−2Ln/c  Eqn. 2where L is the total length of the optical fiber that the pulse travelsfrom the laser diode 1601 to the optical sensor 1615 and back to theoptical receiver 1616 (e.g., photodetector), n is the effective groupindex of the optical-fiber mode and c is the speed of light in a vacuum.The remaining time-difference T is proportional to the differencebetween the FBG's characteristic wavelength and the initial wavelengthλ₀ of the laser diode 1601 (e.g. at the start of the pulse excitation,which also approximately corresponds to the wavelength that is obtainedat nominal current during the continuous wave operation of the laserdiode 1601). This remaining time difference T can be linearlyproportional to the wavelength difference if the pulse-shaping of theDFB diode drive-current is such as to provide a linear wavelength sweepover time, as described previously. Any change in FBG's state (e.g.change in strain and/or temperature) will thus induce change during thetime between initiation of the pulse and its arrival to the opticalreceiver 1616 of the detection system. The actual characteristicwavelength λ of FBG can be thus calculated as:λ=λ₀ +T*Δλ _(p) /t _(p)  Eqn. 3where λ₀ is the initial laser diode wavelength (wavelength at beginningof the pulse), Δλ_(p) is the laser diode wavelength sweep-range, andt_(p) is the pulse duration.

The present measurement system 1600 converts wavelength measuresdirectly into time measures and thus provides an accurate, stable,repeatable measurements. The conversion of optical wavelength measureinto time measures brings several advantages such as the possibilitiesof using simple and cost-effective measure systems that achievehigh-resolution and accuracy at low cost. Such time measure subsystemswith picosecond resolution and accuracy may be provided in the form oflow-cost integrated circuits, e.g. in the form of a time-to-digitalconverter (TDC). One TDC example is the integrated circuit TDC-GP21 orTDC-GPX produced by Acam, Germany. For example, at 300 ns pulseduration, a 10 nm total wavelength sweep-range, and a TDC resolution of10 ps it is theoretically possible to achieve resolution of less thanabout 1 pm (more precisely about 0.33 pm).

In another method embodiment, a wavelength sweep can be also performedduring a cool-down cycle of the active region 102. In this embodiment, ashort duration, high-amplitude pulse is generated. The generated pulsemay have a time duration of about 300 ns, and an amplitude of about 2 A,for example. This pulse heats up the active region of the laser diode.Immediately after termination of this heating pulse, a relatively lowercurrent is driven through the laser diode to keep the diode lasingduring the cool-down phase. The relatively lower current may be typicalnominal drive current specified for DC operation of the laser diode, forexample. In this embodiment, the cool-down process may generate atemperature sweep of the active region and its vicinity, andconsequently the wavelength sweep of the emitted optical power. Thisallows for acquisition of spectral characteristics of an optical sensoror similar optical device during the cooling of the laser diode, asshown in FIG. 17. It is also possible to achieve a substantially linearcooling process by applying proper control of the laser diode drivecurrent during the cooling-down phase.

In another embodiment, as shown in FIG. 18, a measurement system 1800 isshown having a wavelength reference in the form of an added all-fiberFabry-Perot etalon 1805R coupled to the laser diode 1801 to provide forprecise wavelength control. The depicted embodiment includes two moreoptical diverters 1804A, 1804B, such as optical couplers, an additionaloptical receiver 1816R (e.g., a photo-detector), an additionaltrans-impedance amplifier 1817R, an additional decision logic device1818R (e.g., a comparator), and an additional input channel on timemeasures subsystem 1819 (e.g., a TDC) or an additional time measuressubsystem 1819R (e.g., a TDC), as shown. When the pulse on the laserdiode 1801 is released, both TDC channels (e.g., TDC channels) or timemeasures subsystems 1819, 1819R are triggered over the electricalconnections 1828, 1828R by processing unit 1820. The optical pulse thentravels along the optical-fiber 1810 through the first optical coupler1842A, which then divides the optical pulse into two fiber branches 1811and 1811R. The pulse further travels along the length of the opticalfiber 1811R to the second optical diverter 1842B (e.g., coupler), and atsame time along the length of the optical fiber 1811 to the thirdoptical diverter 1842 (e.g., an optical coupler). From the secondoptical coupler 1842B the pulse propagates along the length of theoptical fiber 1812R to the reference all-fiber Fabry-Perot etalon 1805R.The Fabry-Perot etalon back-reflects the incoming pulse, but thisback-reflection is wavelength dependent, i.e. periodic according towavelength. The Fabry-Perot interferometer thus modulates thewavelength-swept pulse into a sinusoidal optical signal, such as thetype shown in FIG. 11.

This reflected sinusoidal signal is guided back to the second opticalcoupler 1842B, which directs part of the reflected pulse power into theoptical fiber 1814R that then guides the pulse to the opticalobservation device 1808R. The optical observation device 1808R mayinclude an optical receiver 1816R, a trans-impedance device or amplifier1817R, decision logic device 1818R, a time measures subsystem 1819R, anda processing unit 1820.

The optical receiver 1816R converts the optical pulse into an electricalsignal that is provided to the trans-impedance amplifier 1817R inelectrical line 1825. The trans-impedance amplifier 1817R amplifies thesignal and transmits the signal over an electrical connection 1826R tothe decision logic device 1818R (e.g., a comparator), which then forms atrain of electrical pulses, and further directs them over an electricalline 1827R to the time measure subsystem 1819R (e.g., a TDC), whichmeasures the time intervals between those pulses generated by thedecision logic device 1818R.

The time measure subsystem 1819R then sends the measured time-intervalvia the data line 1828R to the processing unit 1820, which furthercontrols the parameters for the pulse generator 1806 over line 1822. Ifthe time-intervals between the pulses are substantially equal (period ofpulses is constant) then the wavelength sweep of the laser diode 1801 issubstantially linear. In those cases where the time intervals betweenpulses are different, the processing unit 1820 adjusts the parameters ofthe pulse generator 1806 and consequently the current pulse shape,through signals in electrical line 1823 to the laser diode 1801 in orderto make these distances amongst the pulses approximately equal.Equalization of the distances amongst these pulses linearizes thewavelength versus time dependence of the emitted wavelength. Thewavelength adjusting/linearization system can run independently from therest of the measurement system 1800, which preserves the acquisitionrate of the system. The remainder of the measurement system 1800operates with an optical observation device 1808. The opticalobservation device 1808 may include an optical receiver 1816, atrans-impedance device or amplifier 1817, decision logic device 1818, atime measures subsystem 1819, and the processing unit 1820 operating asdiscussed above for FIG. 16 to interrogate the optical sensor 1805.

Such a measurement system 1800 can achieve improved accuracy overenvironmental influences such as temperature, since the parameters ofthe pulse generator 1802 may be continuously or intermittentlydynamically adjusted through the use of the reference and the closedfeedback-loop to provide linear wavelength sweep of the laser diode1801.

This measurement system 1800 can be embodied in many different ways. Forexample, all three diverter devices 1842, 1842A, 1842B, could be, forexample, replaced by one 3×3 coupler, but the lengths of optical fiber1812R should then be sufficiently different from the length of theoptical fiber 1812 to prevent reflected pulse overlapping (eachreflected pulse that enters the 3×3 coupler is forwarded to all threebranches). In another configuration, an all-fiber Fabry-Perotinterferometer (AFFP) 1805R could be placed in an in-line configurationwith FBG 1805, i.e. along the same fiber line 1812, whilst inserting aproper fiber-delay line between FBG 1805 and AFFP 1805R to separate theback-reflected pulses in time. In the latter case, only one timemeasures system 1819 can be used. Furthermore, instead of activelyadjusting the excitation electrical current pulse's shape, theinformation on the distances between pulses obtained from reference AFFP1805R can be used to perform correction (linearization) of the measuredresults. The AFFP c1805R could also be replaced by a series of FBGs(like 1805), but with different wavelengths for obtaining referencepulses at particular reference wavelengths. However, in that case theadditional FBGs should be isolated from environmental changes.

Furthermore, external temperature control of the laser diode 1801 can beadded to the system of FIG. 18, as shown in the measurement system 1900of FIG. 19, ether to stabilize the wavelength scanning-range and theinitial diode's wavelength and/or to extend the wavelengthscanning-range by further reduction of the environmental temperature ofthe laser diode 1801. For example, laser diode 1801 can be mounted ontoa thermoelectric cooler 1915 (either the case of the diode or diodechip), which can then adjust an operational temperature of the diode1801 via a control signal 1928 from the processing unit 1820. Such aconfiguration provides the opportunity to decrease an initialoperational temperature of the laser diode 1801, which significantlyincreases the wavelength tuning range of the diode 1801. Reduction ofthe laser diode chip's temperature increases the temperature span inwhich the active region temperature can be varied, as previouslyexplained in detail. Lowering the temperature thus extends the activeregion temperature variation span, which leads to a broader wavelengthsweeping-range, as indicated as an example in FIG. 20.

Pulses that produce linear wavelength sweep were first generated by thelaser diode 1801 stabilized at a temperature of 20° C. The wavelengthsweep span was, in this case, approximately 8 nm. Afterwards, the laserdiode 1801 was cooled, while simultaneously increasing the pulseduration. Lowering of the laser diode temperature requires asimultaneous increase in pulse duration and/or amplitude in order toachieve a prolonged wavelength sweep-range. In the particular exampleshown in FIG. 20, the wavelength-sweep of the laser diode 1801 wasextended by cooling the diode to −5° C., while simultaneously extendingthe pulse duration from 300 ns to 600 ns the electrical currentamplitude remaining the same as at 20° C. In such a case the wavelengthsweep span increased by more than 2 nm, to over 10 nm, an approximately20% increase.

In another embodiment, the time measures system 1619, 1819 (e.g., atime-to-digital converter (TDC)) may be replaced by a high-speedanalog-to-digital-converter 2130 as shown in FIG. 21. High-speedanalog-to-digital-converters are widely available at low-cost and caneasily acquire data at rates of more than 500 Msps. At this rate, forexample, the processing unit 1820 can record approximately 100 samplesduring a 200 ns pulse duration. From the recorded samples it is possibleto fully reconstruct the spectral characteristic of the optical sensor1805 and/or the reference sensor 1805R using conventional approximationand interpolation methods. Conventional processors can be replaced byfield-programmable gate array (FPGA) or a similar circuit, which couldfurther improve the performance of such a measurement system. Such anembodiment can provide the possibility of acquiring full spectralcharacteristics of the sensor 1805 by a precise timer recording of thesignals generated by the wavelength sweep, which is particularly usefulduring the integration of more complex spectrally resolved sensors orsensor systems, where peak wavelength observation/determination isinsufficient or when the spectral characteristics of the sensor orsystem take more complex shapes that need to be further analyzed.

In another embodiment, the optical wavelength sweeping apparatus can beused to replace a broad-band optical source such as a superluminescentlight emitting diode (SLED). In this embodiment, the bandwidth of theoptical detection or measurement system 2200 should be (significantly)below the inverse value of the excitation pulse duration in order toallow for averaging of the incoming optical signals. One embodiment ofthe measurement system 2200 is shown in FIG. 22. The measurement system2200 utilizes a wavelength dispersive element 2232 in the form of adiffraction grating or prism as part of the optical observation device2208. A wavelength swept light pulse from the laser diode 2201 (e.g., aDFB laser diode) driven by a current pulse generator 2206, travels alongan optical fiber 2210 through the optical diverter 2204 (e.g., anoptical coupler) and the length of the optical fiber 2212 to a pluralityof sensors 2205A, 2205B, for example. The optical sensors 2205A, 2205Bmay be two or more fiber Bragg gratings each with a different initialwavelength wherein the sensors 2205A, 2205B respond by reflecting backtwo narrow pulses of light that corresponds to the FBG characteristicwavelength of each sensor 2205A, 2205B. Other sensor types may beinterrogated as well.

The reflected pulses are traveling back through optical diverter 2204where part of reflected pulses are divided into the optical fiber 2214,which may have be flat cleaved at an end thereof. Both light pulsesleave the optical fiber 2214 at the flat cleaved end and travel througha collimating lens 2234 which collimates the light beam. The two pulsesof light further travel to the diffraction grating 2232 where each pulseis diffracted at a different angle 236A, 2236B in respect to theirwavelengths. In this manner, the light pulses are received at a specificposition on an optical receiver 2216, which may be a linear detectorarray (e.g., a CCD array). The collimating lens 2234, diffractiongrating 2232, optical receiver 2216 together make up the observationdevice 2208. The acquired data signals are sent to the processing unit2220 via the data line 2225. Suitable conversion, filtering and/oramplification of the data signals may be provided. The processing unit2220 can then calculate the wavelength peak positions of the sensors2205A, 2205B (e.g., FBGs) from the known geometric parameters betweenthe wavelength dispersive element 2232 and the optical receiver 2216(e.g., a linear detector array). In this embodiment, a linearization ofthe wavelength sweep is not necessary, since the spectrum of reflectedlight is directly correlated to the measured parameter. In thisembodiment, the pulse generator 2206 and DFB laser diode 2201 comprisean optical wavelength sweeping apparatus that effectively replaces thebroadband optical source that is usually required in such systems.

Furthermore, the wavelength-swept, short duration, high amplitudeoptical pulses generated by the optical wavelength sweeping apparatus ofthe present invention are ideally suited for use in time multiplexedspectrally resolved sensor measurement systems, such as those that usemultiple FBGs system or FBG arrays.

In an embodiment shown in FIG. 23, multiple optical sensors 2305A-2505F(e.g., FBGs) with different characteristic wavelengths can be locatedarbitrarily close to one another along the same optical fiber 2312. Whenan optical pulse that exhibits a varied wavelength over time arrives atsuch an array of sensors 2305A-2305F, each individual sensor 2305A-2305Fwill reflect back different parts of the incoming optical pulse, thusmultiple back-reflected optical pulses will be generated. The arrivaltimes of each optical pulse consequently represent the characteristicwavelengths of each individual sensor 2305A-2305F (e.g., FBG). Thisoperational regime is well-suited in particular for strain andstrain-related measurements (e.g. force, torque, or the like) as a pairof gratings can be configured in such a way as to exhibit the sametemperature influence but different responses to the strain or otherparameters that can be converted to the strain.

In this embodiment, the time differences between arriving pulses fromthe laser diode 2301, reflected by each sensor 2305A-2305F (e.g., FBG)can be measured to determine strain-induced change, while the totaltravel time measured from the pulse generator 2306 until the arrival ofthe pulses to the observation device 2308 can be correlated to thestrain and the temperature influence, which allows for simultaneousmeasures of strain and/or temperature. The measurement system 2300described herein may be used in other ways known in the art, that allowfor the usage of grating pairs to measure specific target parameters,while canceling out all other influences of paired FBGs.

In many such embodiments, one grating may act as a reference while theother may act as a sensor. However, other possible configurations arepossible. For example, one grating, may expand whilst the othercontracts under a particular measured parameter while all otherenvironmental parameters influence both gratings simultaneously.Accordingly, the direct observation of characteristic wavelengthdifferences provides an excellent way of canceling out unwantedenvironmental influences. The present measurement system 2300 iswell-suited for such sensor measurements since the time measuresubsystem 2319 must only be reprogrammed for measuring time delaysbetween pairs of back-refracted pulses generated by closely spaced pairsof gratings. The other structure of the measurement system 2300 is asbefore described in FIG. 16 and FIG. 19 including optical fibers 2310,2314, optical diverter 2304 (e.g., optical coupler), and observationdevice 2308 including optical receiver 2316 (e.g., a photodetector),amplifier 2317, logic device 2318 (e.g., a comparator), time measuressubsystem 2319 (e.g., a time-to-digital converter (TDC)), and aprocessing unit 2320. The measurement system 2300 may include activecooling by thermally coupling the laser diode 2301 to a cooling device2315 such as a thermoelectric cooler.

In one or more other embodiments, the measurement system 2300 is ideallysuited for the time-domain multiplexing of optical sensors. In thisembodiment, spacing between individual fiber Bragg gratings (FBGs)2405A-2405F should be large enough to prevent potential overlapping ofback reflected pulses, as shown in FIG. 24. The sensors 2405A-2405F(e.g., FBGs) may cover equal or very similar operational wavelengthranges, but sufficient lengths of optical fiber 2412A, 2412B may beinserted in-between the sensor pairs to prevent pulse overlapping. Forexample a “safe” distance D between two FBGs that have the same spectralcharacteristics is determined by generated optical pulse duration T andcan be estimated as:D=(c/n)*T/2  Eqn. 4where c is the vacuum speed of light and n the effective group index ofthe fiber mode. If the pulse duration is, for example, 300 ns, thedistance between the two gratings with equal initial wavelengths shouldbe about at least 30 m. Time-division multiplexed FBGs should also haverelatively low reflectivity and good transmittance to prevent largelosses of optical power. In this case, the processing unit 2320 triggerstime measures subsystem 2319 (e.g., a TDC) at an appropriate time toacquire pulses back-reflected by the observed FBG pair. For example, ifthe length of fiber 2412 is 100 m, the length of fiber 2412A is 200 m,and the pulse duration is 300 ns, the processing unit 2320 shouldtrigger time measures subsystem 2319 with a 3 μs delay, for example, toallow through the reflected signals from the first two sensor 2405A,2405B and to acquire time-difference between the reflected opticalsignals from the FBGs 2405C, 2405D. In order to acquire time-differencebetween the FBGs 2405E, 2405F, processing unit 2320 should trigger atime measures subsystem 2319 with a 6 μs delay, for example. Other timedelays and fiber lengths may be used.

By combining the fast wavelength-sweep achieved by the opticalwavelength sweeping apparatus for the interrogation of FBGs withdifferent characteristics' wavelengths and time division multiplexing asdescribed above, it becomes possible to create relatively larger sensornetworks, which include numerous optical sensors and can cover longdistances amongst individual sensors. By using several optical sourcesat different emitting wavelengths, such measurement systems may befurther expanded.

Due to the relatively large optical power generated by the laser diodewhen driven in a pulsed mode as described herein, a significantextension of the sensor count (a number of sensors coupled to themeasurement system) is possible. As shown in FIG. 25, a measurementsystem 2500 includes an additional multiport diverter device 2540 (e.g.,a multiport coupler such as a 1×N coupler), N 1×2 couplers 2504A-2504N,N optical fiber branches 2512A-2512N containing optical sensors2505A1-2505N4, N optical receivers 2516A-2516N, N trans-impedanceamplifiers 2517A-2517N, N decision logic devices 2518A-2518N (e.g.,comparators), and N time measures systems 2519A-2519N (e.g.,time-to-digital converters or channels of a time-to-digital converter).Once a high-power pulse is generated by the pulse generator 2506 andsent to the laser diode 2501, the short duration, high amplitude pulsetravels along the length of the optical fiber 2510 into a 1×N opticaldiverter 2540, which distributes optical pulses in N optical diverters2504A-2504N through the lengths of the optical fibers 2510A-2510N.

The optical diverters 2504A-2504N operate to distribute light pulsesinto the N branches formed by the lengths of optical fiber 2512A-2512N.Each branch contains a number of optical sensors 2505A1-2505A4 through2505N1-2505N4. The number of optical sensors in each branch is limitedby the wavelength sweep range of the laser diode 2501 as driven by thepulse generator 2506 if the optical sensors have different initialwavelengths or by produced optical power if time domain multiplexing isused. If time domain multiplexing is used, it is desired to ensuresufficient length of optical fibers 2512AF-2525NF between the opticalsensors with equal spectral properties to prevent overlapping of theback-reflected optical signals. The reflected optical signals on eachbranch travel back to the N 1×2 optical diverters 2504A-2504N whichredirect the reflected optical signals to the optical fibers2514A-2514N, which are further connected onto an observation device 2508having the N optical receivers 2516A-2516N. Each optical receiver2516A-2516N converts optical signals into electrical signals, which arefurther amplified by the trans-impedance amplifiers 2517A-2517N 7A-7Cand compared with a reference level using the decision logic devices2518A-2518N (e.g., comparators), which further produce square electricalpulses with the same duration (lengths) as the optical signals received.Each signal from the decision logic devices 2518A-2518N is sent anappropriate channel of the time measures subsystems 2519A-2519N. Ifthere are insufficient time measure channels available in a TDC,additional TDCs can be included within the measurement system to coverall optical branches. This configuration allows for the interrogation ofN optical branches simultaneously, which increases the system dynamicperformance and/or sensor count. Processing unit 2520 and cooling device2515 are as previously described.

Another multichannel embodiment of a measuring system 2600 is shown inFIG. 26 including an optical switch 2645 instead of the 1×N coupler 2540used in the embodiment of FIG. 26. After an optical pulse driven by thepulse generator 2606 and generated by the laser diode 2601 is releasedinto the length of the optical fiber 2610, it then travels through theoptical diverter 2604 and the length of the optical fiber 2610A to theoptical switch 2645. Herein the optical switch 2645 switches between thedifferent optical branches including optical fibers 2612A-2612C andoptical sensors 2605A1-A4 through 2605C1-C4. Each optical branchreflects back the optical signals from the optical sensors 2605A1-A4through 2605C1-C4 to the optical switch 2645 through the length of theoptical-fiber 2610A, through the optical diverter 2604 and the length ofthe optical fiber 2614 into the optical receiver 2616 (e.g., aphoto-detector). Optical receiver 2616 further converts the opticalsignals into electrical signals, as described before, and sends them tothe time measures subsystem 2619, which calculates the time delay thatis correlated to the measured parameter by the processing unit 2620. Theoptical system 2600 in this configuration can only interrogate onebranch at a time via a switch signal in line 2646 to the switch 2645,but does not require additional signal processing components such asphoto-detectors, amplifier, and comparators, and TDCs for each channel,as in previous examples. The remainder of the system 2600 is aspreviously disclosed in the other embodiments.

The above descriptions relate to FBG sensor interrogation. Embodimentsof the present method can be, however, adopted for the interrogation ofother spectrally-resolved sensors, such as Fabry-Perot sensors. Forexample, in the basic system configurations presented in FIG. 14 throughFIG. 18, FBGs can be directly replaced by those Fabry-Perot sensors thathave spectral characteristics that can be resolved by availablewavelength sweep ranges. Other types of optical sensor may beinterrogates by the measurement systems described herein.

Representative Example

An example experimental measurement system as is shown in FIG. 27 wasused to obtain actual experimental data. The measurement system 27 wasconstructed from a programmable pulse generator 2706 using a chain ofeight transistors, as shown in FIG. 14, a standard telecommunication DFBlaser diode 2701 (Osemos model OSMLDP-D513BF2G), and a cooling device2715 that was a thermo-electric cooler to adjust the temperature of thelaser diode 2701. The observation device 2708 included a processing unit2520 in the form of a microcontroller, a time measures subsystem 2719such as a time-to-digital converter type TDC-GP21 (produced by ACAM,Germany), a trans-impedance amplifier 2717, a decision logic device2618, such as a comparator, an optical receiver 2716 such as aphoto-detector. The optical system included an optical diverter 2704such as an optical coupler, respective lengths of optical fiber 2710,2712, 2714 at each branch of optical diverter 2704, and two opticalsensors 2705A, 2705B such as FBGs having two different initialwavelengths.

Both Fiber Bragg gratings within experimental measurement system setupwere positioned 20 cm apart along the optical fiber 2712, and were fixedon the surface of thin metal plate where one sensor 1705A was fixed onthe top side and the second sensor 2705B was fixed on the bottom side ofthe plate, but with both of them were substantially fixed in paralleldirections on the plate. Bending of the metal plate thus caused onesensor 2705A to expand and the other sensor 2705B to compress, which ledto a double bend sensitivity of the measurement system 2700 whileeliminating those environmental parameters on the measure that did notrelate to the bending of the plate (like for example temperature).

In operation, the pulse generator 2706 generated 300 ns long electricalpulses that drove the laser diode 2701 (e.g., a DFB laser diode). Thecurrent versus time characteristics of the electrical pulse was setexperimentally so as to produce optical pulses to provide asubstantially linear wavelength sweep over time. The optical pulsefurther traveled along the optical fibers 2710 and 2712 through theoptical diverter 2704 to the first sensor 2705A. Part of pulse thatmatched the characteristic wavelength of the first sensor 2705A wasreflected back, while the rest of the pulse traveled on to the secondsensor 2705B through the fiber 2712. The other part of the pulse,matching the characteristic wavelength of the second sensor 2705B, wasalso reflected back. Both back-reflected optical pulses were separatedover time to values that were proportional to the FBG's characteristics'wavelength differences and the actual distances between both gratings.Since the wavelength sweep of the laser diode 2701 was approximatelylinear, any change in characteristics' wavelength differences amongstboth FBGs linearly correlates with a change in time difference betweenthe arrival times of both reflected pulses. FIG. 28 illustrates recordedpulses when the detection system 2716-2719 was replaced by a high-speedoptical oscilloscope to record the arriving optical pulses. When thethin metal plate was bent in one direction, the pulses moved apart overtime, while bending of the plate in another direction causes both pulsesto arrive with smaller time-delays.

An example of a method of operation of the electronic part of ameasurement system 2700 (e.g., a sensor interrogation system), in one ormore embodiments, may be presented by the method 2900 shown in FIG. 29.The method 2900 is executed by the processing unit (e.g., 2720).Initially, processing unit 2700 and periphery required for the controlof pulse generator 2706 and time measure subsystem 2700 is initialized2902. Initialization may also include temperature stabilization of thelaser diode 2701, which is performed by thermal control algorithmexecuting in the processing unit 2720 wherein a measured temperature ismeasured against a threshold, for example. In present example, thetemperature of laser diode 2701 was decreased and stabilized at −5° C.,which allows for wavelength sweeps of over about 10 nm.

After initialization of measurement system 2700 and adjustingtemperature of laser diode 2702, the measurement system 2700 starts tomeasure a time difference between the two reflected light pulses thathave been reflected from the sensors 2705A, 2705B. To achieve that,processing unit 2720 may be set in an alert state, where the timemeasurement subsystem 2719 (e.g., TDC) may be triggered in 702 by anincoming pulse to perform time measurement. Furthermore, processing unit2720 starts to generate current pulse by sequentially turning ontransistors (e.g., MOSFET transistors) of the pulse generator 2706,which define total current flowing through laser diode 2701. After allMOSFETs are turned on, processing unit 2720 may turn all transistors toan off state which ends pulse generation. After the optical pulsegenerated by the laser diode 2701 is sent into the optical systemcomprising the two optical sensor 2705A, 2705B, the time measurementsubsystem 2719 (e.g., TDC) may wait for a sufficient number of pulses tobe received (e.g., two pulses for two sensors) in 2708 that have beenreflected from optical sensors 2705A, 2705B. After the two pulses arriveto the time measurement subsystem 2719 (e.g., TDC), internal logic oftime measurement subsystem 2719 may measure a time difference betweenthose respective pulses. This difference may be forwarded to theprocessing unit 2720. Processing unit 2720 may continue to receive dataand may, for example, average the measured time delays (differences) in2910. Numerous time difference samples may be received for averaging,such as a 100 samples. Other numbers of samples may be used.Furthermore, the processed results may be stored in memory and theresults may be displayed in 2912, such as on a suitable screen displayor printout, which can visually present the result, for example in theform of a time chart.

The measured time difference is converted into a measured parameter bythe processing unit 2720 using known wavelength versus time correlationmethods. In this particular embodiment, the correlation corresponded to30 pm/ns. More precisely, before conversation of the measured timedifference to a wavelength difference, the time required to travel thedistance between both FBGs is further subtracted, as already explainedin the previous section.

A thin metal plate was strained using known weights and the results wereobserved. A sample rate of 30 ksps was achieved using the experimentalmeasurement system 2700, with the results shown in FIG. 30. The resultswere averaged to reduce the measurement system bandwidth down to 3 kHz,which additionally increased the system resolution. As seen from FIG.30, the time difference between the pulses changed for 125 ps when theplate was strained in such a way as to induce the FBGs wavelength shiftby approximately 4 pm. The resolution can be further increased byadditional filtering, as shown on FIG. 30. Here, the system bandwidthwas limited to 300 Hz, which can improve spectral resolution to below 1pm.

In summary, embodiments of the present methods and systems describedherein utilize driving high-amplitude, short current pulses through theactive region of a laser diode (e.g., a DFB laser diode) to rapidlyraise or decrease a temperature of the active region. Since laser diodesare generally designed for the constant DC operation, a diode'slife-time when driven under high-amplitude, short current pulsesconditions might be of concern. To test the lifetimes of laser diodesunder such conditions, several laser diodes of type OSMLDP-D513BF2N byOsemos, China, were exposed to pulses of 250 ns in length, a currentamplitude of 2 A, and a repetition rate of 10 kHz. The laser diodes werecontinuously tested for over 6000 hours and no detectable damageoccurred. Accordingly, such a test has proved that the present methodsand systems described herein may be suitable for various applicationswhere low-cost, broad and/or high-speed wavelength sweeps are required.

While the invention is susceptible to various modifications andalternative forms, specific embodiments and methods thereof have beenshown by way of example in the drawings and are described in detailherein. It should be understood, however, that it is not intended tolimit the invention to the particular apparatus, systems or methodsdisclosed, but, to the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the scope ofthe invention.

What is claimed is:
 1. A method of driving a laser diode, comprising:providing a laser diode having an active region; and selectively andrapidly heating the active region and an immediate vicinity by applyingcurrent pulses to perform a wavelength sweep of emitted opticalradiation wherein the selectively and rapidly heating comprises applyinga current pulse amplitude of greater than about five times larger than amaximum declared continuous DC drive current of the laser diode and ashort-duration current drive pulse having a pulse duration of less than50 μs.
 2. The method of claim 1, wherein the selectively and rapidlyheating comprises applying a current pulse amplitude of between about 5times to about 300 times larger than a maximum declared continuous DCdrive current of the laser diode.
 3. The method of claim 1, wherein thepulse duration comprises less than 10 μs.
 4. The method of claim 1,wherein the pulse duration comprises less than 1 μs.
 5. The method ofclaim 1, wherein the pulse duration comprises less than 500 ns.
 6. Themethod of claim 1, wherein the laser diode is a distributed feedback(DFB) diode.
 7. The method of claim 1, wherein the active regioncomprises a thickness of less than 1 μm, an active region cross-sectionof less than 7 μm², and a ratio of active region volume to total laserdiode volume of less than 1/300.
 8. The method of claim 1, wherein theselectively and rapidly heating comprises application of a current drivepulses having a pulse duration and amplitude sufficiently low to preventa rise in an average temperature of the laser chip above 30 degrees C.9. The method of claim 1, wherein an amplitude and pulse duration ofcurrent pulse applied to the laser diode are sufficiently low to preventa rise in an average bulk temperature of the laser diode of more than 2degrees C. after application of a single pulse.
 10. The method of claim1, wherein an amplitude and pulse duration of current pulse applied tothe laser diode are sufficiently high to rise a temperature of theactive region to at least 40 degrees C. at the end of a duration of thecurrent pulse.
 11. The method of claim 1, wherein an amplitude and pulseduration of a current pulse applied to the laser diode are selected toprovide peak optical power at the output of the laser diode that is fivetimes or more higher, ten times or more higher, thirty times or morehigher, or fifty times or more higher than when driven by a declarednominal continuous DC drive current.
 12. The method of claim 1, whereinan amplitude and pulse duration of the current pulses applied to thelaser diode are selected in a way to provide a ratio of a peak opticalpower at a beginning of the pulse to a minimum optical power at the endof the pulse duration of less than 15:1, or even less than 10:1.
 13. Themethod of claim 1, wherein the selectively and rapidly heating comprisesapplication of a drive current pulse having changing amplitude over aduration of the current pulse.
 14. The method of claim 13, wherein thedrive current pulse comprises an amplitude change of greater than 25%.15. The method of claim 1, wherein the wavelength sweep of emittedoptical radiation comprises a substantially linear wavelength changeversus time.
 16. The method of claim 1, wherein a duty ratio of pulsesapplied to the laser diode are less than 1/10.
 17. The method of claim16, wherein the duty ratio comprises less than 1/30.
 18. The method ofclaim 1, comprising completing a wavelength sweep in less than about 50μs.
 19. The method of claim 1, wherein the wavelength sweep comprises awavelength range of greater than about 5 nm.
 20. The method of claim 1,wherein the wavelength sweep comprises a wavelength range of greaterthan about 10 nm.
 21. The method of claim 1, wherein the wavelengthsweep comprises a wavelength range of greater than about 10 nm in lessthan about 300 ns.
 22. The method of claim 1, comprising cooling thelaser diode.
 23. An optical wavelength sweeping apparatus, comprising: alaser diode having an active region and an immediate vicinity; and apulse generator coupled to the laser diode configured and operable toprovide current drive pulses to the laser diode to selectively andrapidly heat the active region and the immediate vicinity and produce awavelength sweep of emitted optical radiation wherein the selectivelyand rapidly heating comprises applying a current pulse amplitude ofgreater than about five times larger than a maximum declared continuousDC drive current of the laser diode and a short-duration current drivepulse having a pulse duration of less than 50 μs.
 24. The apparatus ofclaim 23, wherein the wavelength sweep of emitted optical radiationcomprises a wavelength range of greater than about 10 nm in less thanabout 300 ns.
 25. The apparatus of claim 23, wherein the current drivepulses comprise a pulse duration of less than 10 μs.
 26. The apparatusof claim 23, wherein the current drive pulses comprise a pulse durationof less than 1 μs.
 27. The apparatus of claim 23, wherein the pulseduration comprises less than 500 ns.
 28. The apparatus of claim 23,comprising multiple laser diodes with different initial wavelengthsand/or wavelength spans.
 29. The apparatus of claim 23, comprising areference sensor coupled to the pulse generator.
 30. The apparatus ofclaim 23, comprising a cooler thermally coupled to the laser diode. 31.The apparatus of claim 23, comprising multiple optical fiber branchescontaining optical sensors.
 32. An optical measurement system,comprising: a laser diode having an the active region and an immediatevicinity; a pulse generator coupled to the laser diode configured andoperable to provide current drive pulses to the laser diode toselectively and rapidly heat the active region and the immediatevicinity to produce a wavelength sweep of emitted optical radiationwherein the selectively and rapidly heating comprises applying a currentpulse amplitude of greater than about five times larger than a maximumdeclared continuous DC drive current of the laser diode and ashort-duration current drive pulse having a pulse duration of less than50 μs; an optical diverter coupled to the laser diode; an optical sensorcoupled to the optical diverter; and an observation device coupled tothe optical diverter.